1. Technical Field
The field of art in which this invention pertains is that of an extrusion system and method particularly for use in extruding precision strips of rubber or elastomeric compounds with profiles of very accurate dimensions in a trouble free and essentially automated operation, and which enables rapid changeover between various types of such compounds. More particularly, the invention relates to such an extrusion system and method in which equilibrium conditions are reached rapidly during start up and after compound changes enabling the economic production of strip material, primarily for use in the subsequent manufacture of tires, air springs and similar items that include rubber compounds.
2. Background Information
Presently various extruders are used to produce rubber or elastomeric extrudates of different shapes. In spite of improvements in equipment design made over the past years, extrudate gauge and weight variations of more than 4% are typically encountered and significant gauge changes occur when rubber feed strips of different composition, stock viscosity and other properties including surface friction are used. The extrudate temperature typically is more than 120.degree. C., particularly when the equipment is operated with an extrusion head and die requiring high pressures and at higher extrusion rates. Furthermore, when conventional strainers are used in line with an extruder, further temperature increases are induced due to the high pressure drop across the strainer screen and its support system. This, in turn, requires extrusion rates to be lowered up to 40% to keep the extrudate temperature from exceeding certain set limits. This temperature must be maintained at a predetermined level to prevent partial vulcanization of the rubber compound as it moves through the extruder, strainer, extrusion head and other components of the system to prevent imperfections in the final extruded strip.
Various types and elements of precision extrusion systems are known and have been used for the processing of a variety of plastics and fibers. However, the chemical make-up and mechanical properties of plastic and fibers and rubber or elastomeric compounds are completely different whereby the use of the various equipments and processes in the plastic and fiber industry is not compatible with or usable in the rubber industry and visa versa, since each technology has its own special problems and desired results to be achieved by a particular extrusion system and method. In a rubber extrusion system, the rubber being extruded will be of varying molecular weight and may contain a high concentration of fillers which can cause them to be very abrasive and can have a wide variety of viscosities. Furthermore, the makeup of rubber strips introduced into an extruder can vary appreciably. In a cold feed extruder system, the ambient temperature of the rubber can vary between 15.degree. C. and 50.degree. C. In a hot feed extruder on the other hand, the rubber strips may have a temperature of between 80.degree. C. and 110.degree. C. Therefore, since the feed compound in a rubber extrusion system is completely different than the feed material in a plastic extrusion system, that which works in the plastic extrusion system will not necessarily work in a rubber extrusion system.
Also, rubber will cure at relatively low temperatures and cannot tolerate high temperatures during the processing and extrusion thereof. Plastics generally are extruded at very high temperatures, for example, between 220.degree. C. and 250.degree. C. without any degrading or adverse effect on the plastics. When extruding rubber it is desirable to maintain the temperature of the rubber as low as possible, preferably at 100.degree. C. or less. This lower extrusion temperature of the rubber is desirable since it allows the rubber compound to be formulated so that the onset of vulcanization occurs at a lower temperature which in turn reduces the time and cost of vulcanization. Lower temperature extruded rubber also will enable shorter cooling conveyor lengths to be utilized reducing equipment costs and processing time. Also, the lower temperature of the final extruded product will diminish the dimensional changes occurring in the extrudate upon leaving the extrusion die. However, during the extrusion of rubber compounds, heat buildup occurs due to shear and frictional heat generated during the screw extrusion of the rubber which is not a problem in the extrusion of plastics and fiber.
Thus, in the extrusion of rubber compound, it is desirable to achieve an accurate dimensional stability of the extrudate and low extrudate temperatures while straining and processing at high throughput rates. One of the means which facilitates the meeting of this objective is to use a long extrusion die land. However, this longer die land requires a high head pressure which in prior extrusion systems results in undesirable higher extrudate temperatures.
In the extrusion of rubber compound strips, in addition to the excess heating problems discussed above, another common problem is that, depending upon the origin of the raw rubber and rubber compounds supplied to the systems, a variety of foreign materials are present in the rubber which must be removed before the final extrusion of the strip. Heretofore, this required the use of separate screening and straining procedures to ensure that the rubber is relatively free of such foreign materials before entering it into the extrusion system. Various types of straining equipment have been devised for removing such foreign materials, but these result in a considerable pressure drop and temperature rise as the material moves through the strainer. Many of these problems have been eliminated by the construction of a low pressure drop strainer of the type shown in U.S. Pat. No. 4,918,017, which is also assigned to the assignee of the present invention.
Another problem with prior rubber extrusion systems, dissimilar to those in the plastic and fiber extrusion systems, is cleanout. It is desirable in rubber extrusion systems and methods that a variety of rubber compounds of various characteristics be used sequentially for manufacturing a variety of products. Heretofore, this required shut down of the equipment and a subsequent removal of the remaining compound before a new rubber compound could be moved through the system since the various components of the extrusion system are bolted together requiring hours to disassemble and clean. In the plastic and fiber extrusion industry, to avoid this extensive downtime, it is common practice to flush the components for several minutes with the new polymer and scrap the flushing extrudate. Such a flushing practice is not acceptable in the rubber extrusion industry because it is extremely slow and expensive, especially when a change in rubber compound may occur numerous times throughout a work shift. The only practical solution is to disassemble the components and to remote the rubber stock from the interior of the equipment.
Moreover, the tire industry often requires that smaller lots of differently shaped rubber or elastomeric strips are extruded, requiring frequent compound and die changes throughout the production day. Therefore, it is critical, in order to achieve economic production, that the time to reach the required dimensional specification of the particular configured extrudate passing through the die, must be as short as possible. This requires that the operating pressure and temperature at the die be reached and stabilized as quickly as possible which is difficult with an usual auger type extruder.
None of the above listed problems found in the extrusion of rubber compounds are a problem or are of much concern in the extrusion of plastics and fibers. Also, gear pumps to date, have not been successfully used in rubber extrusion, although the same have found some success in the extrusion of plastics and fiber.